An internal combustion engine (hereinafter, simply referred to as “engine”) needs to be started by using a starting apparatus (hereinafter, pertinently referred to as “starter”). As the starting apparatus, there are a gear type starter and the like all of which are common in that an electric motor (hereinafter, simply referred to as “motor”) constitutes a drive source.
In starting the engine, comparatively large torque is required although depending on a kind and a displacement thereof. Therefore, when the engine is cranked directly by the motor, the physical configuration of the motor naturally becomes large. Hence, in a recent starting apparatus requesting light-weighted compact formation, high torque necessary for starting is achieved by interposing a reduction gear between the motor and the engine to thereby increase a speed reducing ratio.
Although there are various reduction gears, a reduction gear of a planetary gear type, which compactly achieves a large speed reducing ratio, is frequently used. In the planetary-type reduction gear, a driving force inputted from the motor to a sun gear of the reduction gear is outputted from a carrier supporting a planetary gear with high torque. As premise thereof, a large reaction force (torque) produced in accordance with the output needs to be received by an internal gear. That is, rotation of the internal gear in the circumferential direction needs to be restricted.
Meanwhile, in the engine, torque necessary for rotating the engine is rapidly varied by strokes of intake, compression and the like and an engine rotation speed is also pulsated. The motor of the starter cannot well follow such a load variation or the like and therefore, the reaction force applied to the internal gear is not constant. As a result, unpleasant sound is likely to be generated in starting the engine due to vibration or the like of the internal gear by simply constraining the internal gear.
Hence, in order to alleviate and absorb the load variation or the like operated to the internal gear or the like, a shock absorbing member having an elastic body of rubber or the like is arranged between the internal gear and a portion of constraining the internal gear. This is for example proposed in unexamined Japanese Patent Publication No. JP-A-5-52166.
According to the starter disclosed in JP-A-5-52166, a projection, which extends from a side face of the internal gear in the axial direction, is held by a shock absorbing member (elastic body). Therefore, the starter is not enlarged in a radially outside direction. In this case, however, a friction plate is biased toward a pivoting plate engaging with the internal gear and the internal gear is constricted by friction force produced between them. Therefore, the structure of the starting apparatus is complicated. Further, the internal gear is not provided with a detent and therefore, the internal gear continues rotation little by little while the exerted reaction force is large. Therefore, an efficiency of transmitting drive force of the motor is likely to be reduced. If the detent is simply provided to the internal gear, a large reaction force is applied to the detent member. This may cause damage to the periphery of the detent.